The present invention relates to a method of removing a contaminated surface layer by melting or evaporating the same with a laser beam applied to a contaminated member (an object to be decontaminated). Although the range of application of the techniques concerning this method is not specifically limited, they are effective, for example, in removing a radioactive contaminant from a surface of a member placed in such a radiation environment as in a nuclear reactor, and thereby cleaning (decontaminating) the contaminated member.
In various kinds of nuclear facilities, there are equipment, machines and tools exposed to radiation and coated at the outer surfaces thereof with a thin layer of a radioactive contaminant. The presence of such a layer of a radioactive contaminant places restrictions on workers, who make inspections of and carry out maintenance and repair of such equipment, machines and tools, with respect to various matters including operation time. In order that such operations can be carried out safely, a method of removing a radioactive contaminant from the surfaces of the equipment, machines and tools has been developed. One example of such a method is a laser decontamination method in which a contaminated surface layer is removed by melting or evaporating the same with a laser beam applied thereto.
An example of a prior art laser decontamination method is illustrated in FIG. 6. A condenser lens 12 is incorporated in a gas supply nozzle 10, and a gas is supplied to a contaminated material 14 as the gas supply nozzle 10 is moved along and relatively to a surface of the contaminated material 14 with a laser beam applied thereto. The gas supply nozzle 10 is formed of a cylindrical member gradually narrowed toward a free end portion (lower portion in FIG. 6) thereof, and a plane 16 of an end surface of an opening at the free end of the nozzle is opposed to the contaminated member 14 in parallel therewith. The axis (vertical axis) 10a of the gas supply nozzle 10 agrees with an optical axis 18a of a laser beam 18. A side wall of gas supply nozzle 10 is provided with gas introduction ports 20, 21, through which a gas is supplied to the interior of the gas supply nozzle 10. The advancing direction of the gas supply nozzle 10 (laser beam scanning direction) with respect to the contaminated member 14 is shown by an arrow S.
When the laser beam 18 condensed by the condenser lens 12 is applied to the contaminated material 14, the portion of a contaminated surface layer of the contaminated member 14 to which the laser beam is applied is heated rapidly, and melting and/or evaporation of the surface layer occur. When the molten material or the evaporated material is scattered or moved, the contaminated surface layer is removed, i.e., the laser beam-applied portion 14a of the contaminated material 14 is decontaminated. During this decontamination operation, the gas supplied from the plane of the opening 16 of the nozzle 10 works to spatially control the molten material or evaporated material (works to move the molten material or evaporated material from the laser beam-applied portion 14a to the outside by the kinetic energy of the gas molecules or by a dynamic pressure of the gas), and to prevent an oxidation reaction from occurring in the laser beam-applied portion 14a.
The laser beam 18 on the surface of the contaminated member is in the shape of a dot when the condenser lens 12 comprises a spherical lens, and becomes linear when the condenser lens 12 comprises a cylindrical lens. Therefore, when the contaminated member 14 is scanned with the laser beam 18 or the contaminated member 14 is moved, a linear or planar decontaminated surface can be obtained respectively. Since a high decontamination area processing rate is demanded generally, a cylindrical lens is used as the condenser lens 12 in many cases.
When the laser beam 18 is applied to the surface of the contaminated member 14 by such a method as described above, a molten layer on the surface of the contaminated member 14 is forced out toward the circumference of the laser beam-applied portion due to the influence of an evaporation reaction force, and the scattered molten substances and evaporated substances are deposited again on the surface of the contaminated member 14. Therefore, the profile of the surface of the laser beam-applied portion becomes as shown in FIG. 7A in many cases. Namely, a projection 22 occurs around the laser beam-applied portion.
When the contaminated member 14 in this condition is scanned with the laser beam or moved as previously described, this phenomenon occurs repeatedly, so that the profile of the surface of the laser beam-applied portion becomes as shown in FIG. 7B. Namely, the degree of unevenness of the surface of the laser beam-applied portion becomes high, and an effective decontamination depth cannot be obtained. Since the molten substances and evaporated substances are contaminated, the laser beam-applied surface influenced by the recoagulation and redeposition of these substances is not sufficiently decontaminated.